Optimization of X - ray Radiographic Imaging

Synopsis This chapter presents x-ray image quality optimization techniques using digital phantom images and computerized observer models. The X-ray imaging parameters are optimized so as to maximize tumor detectability while minimizing the patient dose. Digital phantoms are generated using the virtual photographic and radiographic imaging simulator (ViPRIS) package. ViPRIS uses a tomographic phantom, VIP-Man, constructed from Visible Human anatomical color images to simulate the scattered portion using the ESGnrc Monte Carlo code. The primary portion of an x-ray image is simulated using the projection ray-tracing method through theVisible Human CT data set. To produce a realistic image, the software simulates quantum noise, blurring effects, lesions, detector absorption efficiency and other imaging artefacts. The primary and scattered portions of an x-ray chest image are combined to form a final image for computerized observer studies and image quality analysis. Absorbed doses in organs and tissues of the segmented VIP-Man phantom are obtained from the Monte Carlo simulations. Tumor detectability is determined by Receiver Operating Characteristic (ROC) methodology. Hotelling and Laguerre–Gauss Hotelling observers are used to perform various lesion detection tasks. Several model observer tasks were used including SKE/BKE, MAFC and SKEV. The energy levels and fluence at the minimum dose required to detect a small lesion were determined with respect to lesion size, location and system parameters. The purpose of combining computerized observers and digital phantoms is to develop quantitative image quality metrics and simulate human subjects 2 in order to analyze a volume of data that would otherwise be impossible to handle with actual human subjects. In the United States, approximately 250 million radiological examinations are performed each year, making diagnostic medical examinations the largest source of man-made radiation exposure (UN 2000). A major goal of radiography is to maximize the amount of diagnostic information while minimizing the radiation exposure to the patient. All radiographic x-ray examinations require the selection of beam parameters, which affect both the patient dose and the corresponding image quality. Optimization is difficult because of the following reasons: (i) The x-ray imaging chain contains a large number of variables associated with the x-ray beam characteristics, patient anatomy, scatter removal grid, and x-ray detector system. (ii) Diagnostic x-ray examinations involve various body sites and tasks. (iii) Effective dose is the sole indicator of patient risk but is not obtainable without a whole-body phantom of delineated organs. (iv) The images must be evaluated by an observer with respect to …